A key component in current optical networks is the waveguide grating router described in U.S. Pat. No. 5,136,671, issued on Aug. 4, 1992, and entitled “Improved Optical Switch, Multiplexer and Demultiplexer”. This router is currently used in optical networks to increase the long distance capacity of optical fibers by increasing the number of wavelength channels simultaneously transmitted in each fiber. Typically, in optical networks using Dense Wavelength Division Multiplexing (DWDM) the order of each router is larger than 30, and the grating can then be realized by simply using a symmetric arrangement of two identical sections A and B, equally contributing to the order of the grating as shown in FIG. 1. On the other hand, in Local Access Networks using Course Wavelength Division Multiplexing (CWDM) the order can be appreciably smaller than 30, and the grating geometry must then be modified, as shown for instance in U.S. Pat. No. 5,212,758, issued on May 18, 1993. In that patent the grating comprises two sections A and B of opposite curvatures, and an additional section C is included between A and B as shown in FIG. 2. A disadvantage of this arrangement, however, is that section C substantially increases the size of the grating, thus increasing loss and crosstalk, and reducing the maximum number of devices that can be included in each wafer.